When existing users log in, it screws up my signup form

1. I have a signup form (https://www.aerocinema.com/amember/signup/QzgIWcsEl/) with an embedded script that picks up the name/lastname and email and feeds that into a 3rd party contact manager (SharpSpring) that my company uses for sending out email/campaigns.

2. I want everyone who goes through a specific signup form to trigger that script. So far, so good.

3. But if you are existing member, instead of creating a new account you can use aMember's login feature to log in (big green button in my case). This brings a pop-up window.

4. For existing members, When you enter your username/pass in the pop-up window the page redirects you forward to the next step without triggering that 3rd party form. This is the problem. Now I don't know that you have gone through the page. Only new users get added, existing users are invisible because they don't trigger the embedded script.
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So what I am wondering is....how can I get everyone, even existing users, to trigger that embedded script? If I also add it into the code of the pop-up window itself, anyone who uses that ANYWHERE on the website will trigger it. Not good.

-Is there a way to put the "login if you are already a user" feature on the same page as the signup page, instead of a pop-up window? I mean that entering your credentials takes place on the same page.

-Or maybe I can put the "login if you have an account" step BEFORE even getting to the signup page, so that your name and email are already known and therefore get fed into the embedded script.

I've been trying to get my head around this for a few days. Any advice would be very welcome.